Process and apparatus for heat treating aluminum ingots



PROCESS AND APPARATUS FOR HEAT TREATING ALUMINUM INGOTS Filed April 1, 1965 June 4, 1968 T. w. F. FOSTER ET AL 2 Sheets-Sheet 1 l l I i INVENTORY THOMAS W. F. FOSTER BY JOHN B. ELSEE June 1968 T. w. F. FOSTER ETAL 3,386,717

PROCESS AND APPARATUS FOR HEAT TREATING ALUMINUM INGOTS Filed April 1, 1965 2 Sheets-Sheet 2 EMal 2e 2a m E 27 27 -28 \2 26 'B 29 3o\ -;ao g F1g 3' EXHAUST PORT INVENTORS THOMAS W. F. FOSTER BY JOHN B. ELSEE United States Patent 3,386,717 PROCESS AND APPARATUS FOR HEAT TREATING ALUMINUM INGOTS Thomas W. F. Foster, Walnut Creek, Calif., and John B. Elsee, Spokane, Wash, assignors to Kaiser Aluminum & Chemical Corporation, Oakland, Calif., a corporation of Delaware Filed Apr. 1, 1965, Ser. No. 444,602 15 Claims. (Cl. 263--28) The present invention is related to heating of massive bodies, such as ingots, billets and the like of aluminum and its alloys. More particularly, the invention involves a method of and apparatus for heating or heat treating ingots or billets of aluminum and its alloys in a direct fired furnace without impairing the metal surfaces by deleterious water staining, excessive surface oxidation or high temperature oxidation resulting therefrom.

In the aluminum industry the heat treatment of ingots for either hot working or homogenization is presently done principal-1y in either molten salt baths or air furnaces using radiant gas tubes. Induction coils and electric resistance furnaces are also sometimes used to heat the ingots, but electrical costs and the capital costs of the necessary equipment have proven to be too high for such systems to enjoy wide usage. It has long been known that it would be very eificient to heat treat the ingots of aluminum and its alloys in direct fired furnaces because of the short time required to heat the ingot up to temperature. Such a method of heating ingots has not been used extensively because it was believed that in designs that allow products of combustion to come in contact with the work, high temperature oxidation of the ingot is apt to occur.

The aluminum oxide surface layer on ingots which is formed by heating the ingots to a temperature between 700 and 900 F. is believed to be amorphous in character. It is known that aluminum oxide formed in heating to this temperature range has a high adsorptive power for water vapor. The adsorption of water vapor onto the aluminum oxide film on the ingot can leave a stain on the surface of the ingot which is diflicult to remove. This stain gives the ingot an unsightly appearance which is deleterious in that an subsequent rolling of the ingot to sheet or plate, the stain harms the highly desirable bright, uniform in appearance, surface of the met-a1.

An even more serious problem in the heat treating of ingots of aluminum and its alloys in direct fired furnaces which is also related to the presence of water vapor is the possibility of high temperature oxidation of the work resulting therefrom.

The exact mechanism of high temperature oxidation does not appear to have been determined at present. However, on the basis of the discussion of the problem in the Metals Handbook, Volume Two, Eighth Edition, 1961, by the American Society for Metals at page 278, and also the article Atmospheric Control of the Heat Treatment of Aluminum Products by P. T. Stroup in the book Controlled Atmospheres by the American Society for Metals, 1942, it appears that the condition may be due to moisture serving as a source of oxygen and nascent hydrogen which diffuses into the metal when the aluminum is exposed to a moisture containing atmosphere at high temperatures. The most common manifestation of high temperature oxidation is surface blistering but frequently the attack is subsurface in the form of internal discontinuities and voids which can be detected only by careful ultrasonic inspection or metallography. This subsurface attack and the resulting condition of the metal usually cannot be tolerated due to the general market demand for high quality aluminum materials having a bright, shiny, uniform in appearance, surface. Although this type of oxidation can occur in all types of aluminum "ice and its alloys, alloys of aluminum with copper or with magnesium appear particularly vulnerable to this type of attack.

It has also been known that the presence of small quantities of sulfur dioxide in a moisture containing atmos phere would greatly accelerate the attack. One theory with respect to this is that sulfur compounds functions as decomposers of the natural oxide surface film, eliminating it as a barrier either between the moisture and the aluminum or between the nascent hydrogen and the aluminum.

The problem is further compounded by modern heat treating practices wherein the ingot surfaces are scalped or machined prior to heating. The scalping operation removes the normal protective oxide film from the aluminum, as well as some metal, exposing a freshly cut aluminum surface which is more susceptible to high temperature oxidation than a surface from which the film has not been removed.

Because of these problems, the heat treating of ingots of aluminum and its alloys in direct fired furnaces has not been considered practical. The Metals Handbook, volume two, at page 278 states that it is usually unsatisfactory. On the other hand, it has been known that prolonged exposure of the ingot to elevated temperatures in a furnace of any design would result in a heavier oxide film being formed on the ingot. This heavier film will also detract from the appearance of the ingot.

The present invention involves the heating to heat treat ingots of aluminum and its alloys in a direct fired furnace without water staining of the ingot or high temperature oxidation of the ingot resulting therefrom. The method of the present invention involves heating the ingot out of contact with combustion product gases until at least the surface temperature of the ingot is above the dew point of the water vapor in the combustion product gases, thereafter heating the ingot in a direct fired furnace in contact with combustion product gases: to the heat treat temperature, and holding the ingot at the heat treat temperature for the desired period of time.

It is a further object of this invention to provide apparatus for the heat treating of ingots of aluminum and its alloys in a direct fired furnace as mentioned above. The apparatus of the present invention involves means for heating the ingot out of contact with combustion product gases until at least the surface temperature of the ingot is above the dew point of the water vapor in the combustion product gases, means for then transferring the ingot to a direct fired furnace or zone, the .ingot being heated in the direct fired furnace to the heat treat temperature in contact with combustion product gases and means for holding the ingot at the heat treat temperature for the desired period of time.

In the accompanying drawings, which are presented for illustration and are not to be construed as limiting the invention, there is shown suitable apparatus in which this invention may be carried out.

In the drawings:

FIGURE 1 is a diagrammatic sketch showing in elevation one form of apparatus embodying the principles of this invention;

FIGURE 2 is a horizontal section through another form of apparatus embodying the principles of this invention taken on a plane just below the furnace roof;

FIGURE 3 is a front elevational view of a direct fired furnace embodying the principles of this invention with the door removed for purposes of clarity;

FIGURE 4 is a front sectional view in elevation taken at the center of the direct fired furnace shown in FIG- URE 3.

Basically, and with reference to the FIG. 1, the invention involves heating the ingot 11 out of contact with combustion product gases in a pre-heat furnace 12 until the surface temperature of the ingot is above the dew point 'of water vapor in the combustion product gases, thereafter, heating the ingot 11 in a direct fired furnace 13 in contact with combustion product gases to the desired ingot temperature and finally holding the ingot 11 at this heat treat temperature for the desired period of time. Heaters are not shown in FIG. 1 as the initial heating of the ingot out of contact with combustion product gases until the surface temperature of the ingot is above the dew point of water vapor in the combustion product gases may be accomplished in many ways. For example, pre-heat furnace 12 may be a radiant tube type warming oven furnace. Another way of heating the ingot until its surface temperature is above the dew point of water vapor is to have pro-heat furnace 12 be an induction furnace. Still another way is to heat furnace 12 by direct firing until the ambient temperature in furnace 12 is quite high, cutting the burners and then placing the ingot 11 in the furnace until its surface temperature is above the dew point of the water vapor in the combustion product gases.

It is important to note that it is not necessary for the entire mass of the ingot to be heated above the dew point of the water vapor. Only the surface portion of ingot 11 need be heated to a temperature above the dew point of the water vapor. This will prevent water from condensing out on the surface of the ingot and forming a stain as it permeates through the amorphous oxide film present thereon. In practice, the internal portion of the ingot may and does remain quite cool.

The ingot is next heated in a direct fired furnace 13 in contact with combustion product gases to the desired ingot temperature. Heaters are not shown in FIG. 1 as this heating may be done in a special direct fired furnace, in the same furnace in which the preheating of the surface of the ingot to above the dew point was done or in a different section of the same furnace in which the preheating was done, depending upon the particular arrangement of apparatus and method of preheating the ingot above the dew point. For example, if an induction heater had been used in pre-heat furnace 12 for preheating ingot 11 above the dew point, it would then be necessary to pass ingot 11 into a direct fired furnace 13 for heating the ingot to the desired temperature. This might involve transferring the ingot into a separate furnace. In a continuous furnace as shown in FIG. 1, wherein the warming zone or pre-heat furnace 12 and the direct fired heating zone or furnace 13 are juxtaposition, ingot 11 would be passed by suitable means as endless conveyor 14, from pre-heat furnace 12 to direct fired furnace 13 through an air curtain 15 or other suitable gas sealing means. Similarly, if a radiant heat type warming oven had been used it would be necessary to pass the ingot to a separate direct fired zone or separate direct fired furnace 13 as shown in FIG. 1. If the preheating above the dew point had been done in a furnace which had been initially warmed up by direct firing, the fuel then being cut off while the ingot was passed in and preheated to just above the dew point, one could simply turn the fuel back on in that same furnace zone and reignite it. Natural gas is the preferred fuel for the direct firing, however, the invention is not limited to the use of natural gas. In fact, any fuel low in sulfur, if properly combusted, is suitable for this process. Most of the natural gas supplies in commercial use in this country result in a combustion product gas wherein the dew point of the moisture content is between 130 and 150 F.

In heating the ingot in the direct fired furnace, the ambient temperature in the furnace can be carried relatively high because of the high internal heat transfer rate of the aluminum metal ingot. Thus, a very fast heat up of the ingot can be obtained. As shown in Table I, the heat up time for an ingot in bringing it to hot working temperature in heat to roll practice is in the neighborhood of 19-24 hours. In a direct fired furnace, this heat up time can be reduced to 2 hours. Because of this relatively short cycle, the surface oxidation of the ingot is greatly reduced. The preheating of the ingot above the dew point before exposing it to the direct fired atmosphere results in a shiny ingot surface free of water stains. It has also been found that the ingot is free of high temperature oxidation.

After the ingot has been raised to the desired heat treat temperature in a direct fired furnace, it is held at this elevated temperature for the desired soaking period. For heat to roll practice, the soak period is relatively short. The instant process is also suitable for homogenization practice wherein the soak period is substantially longer as the ingot is held at the elevated temperature long enough to make it uniform in composition by eliminating coring and concentration gradients. The ingot 11 may be held at the elevated temperature in furnace 13 as shown in FIG. 1. In this case, there would be great heat input in the initial part of the furnace 13 and very little input in the final or holding zone of furnace 13. Furnace 13 can be made long enough for the heat treatment to be completed therein and ingot 11 can then be passed directly to the rolling mills with no unnecessary holding time or separate holding furnaces. The ingot 1.1 may also be held at the elevated temperature for the desired soaking period in a separate holding furnace, or a separate zone of the same furnace, in which case the holding furnace need not be direct fired but may be heated by any suitable means. In all cases, the ingot passes through the heat treat cycle free of deleterious water staining, heavy surface oxidation, and high temperature oxidation.

From these observations, it appears that, even when the ingot is held for a long period of time at a high temperature as in homogenization practice, the bent up period is critical for these attacks. The initial preheat above the dew point prevents water staining from occurring and the very rapid heat up to temperature in the combustion product gas atmosphere of the direct fired furnace prevents a heavy surface oxide coating from forming and also prevents high temperature oxidation from occurring.

The soaking period for the ingot may be carried out in a direct fired furnace, however, it is not necessary that this be done. In certain instances, it may be desirable to pass the ingot to a different furnace or a different zone of the same furnace for the soaking period in which case radiant heating or induction heating or any other suitable method of heating the ingot may be used for the soak period.

Although the various zones of the furnace are shown arranged linearly in FIG. 1, it is to be understood that other arrangements of the various zones in juxtaposition are possible. For example, a circular arrangement may be used. FIG. 2 shows such a circular furnace. With reference to FIG. 2, ingot 11 is loaded by any suitable means not shown onto circular endless conveyor 16. In-

g0t 11 is then carried by conveyor 16 through the preheat furnace zone 17 where it is heated by any suitable means 18 as previously discussed until the surface temperature of the ingot is above the dew point of the water vapor in combustion product gases. Ingot 11 is then carried by conveyor 16 through suitable sealing means as air curtain 19 into direct fired furnace zone 20. In direct fired furnace zone 20 the ingot is heated in contact with combustion product gases from burners 21 to the desired ingot temperature. Ingot 11 is then carried by conveyor 16 to holding furnace zone 22 where it is held at this heat treat temperature for the desired period of time. If desired, suitable sealing means as air curtain 23 may separate holding furnace zone 22 from direct fired furnace zone 20. Suitable heating means 24 may be used to maintain the desired temperature in holding furnace zone 22. Ingot 11 is then carried from holding furnace 22 by conveyor 16 and unloaded from conveyor 16 by any suitable means not shown.

Clad ingots which have a core of aluminum or an alloy thereof and a surface layer of the same or a diflerent metal affixed thereto to subsequently produce a composite metal having the layers bonded together as by a co-rolling, may also be heat treated in accordance with this invention, but special precautions are necessary. In clad ingots, the air gap between the liner and the core not only impedes heat passage from the furnace atmosphere to the ingot but concentrates the heat in the liner causing a wide temperature difference between the liner and the core during heat up. It has been found that if the ambient temperature in the furnace is carried too high, the cladding will melt. Therefore, it is necessary that the ambient temperature be kept below the melting point of the cladding material while the ingot is being heat treated. No further problems then occur, although the heat up period for clad ingot is longer than that for nonclad ingot due to the lower ambient temperature and lower heat transfer rate. However, as shown in Table I wherein a 2024 alloy core clad with a 1230 alloy liner was heat treated in accordance with this invention, a substantial reduction in the time required to bring the ingot to hot working temperature still occurs. The ingot is free of water staining, heavy surface oxidation and high temperature oxidation. No bonding problems were encountered when the ingot was subsequently hot rolled.

Table I lists typical alloys of aluminum which have been heat treated in accordance with this invention and shows the substantial time savings resulting therefrom. In all cases, the ingot was free of water stain, excessive surface oxidation and high temperature oxidation. In view of these tests, both in heat to roll practice and homogenization heat treat practice, it would appear that alloys of aluminum and other constituents such as magnesium, manganese, copper, silicon, iron chromium, zinc, and titanium may all be heat treated in direct fired furnaces in accordance with this invention and the beneficial results will occur. The alloy designations used are those shown in the above mentioned Metals Handbook and promulgated by the Aluminum Association.

TABLE I HEAT TO ROLL-SOAK TEMPERATURE 850 F.

Direct Average Fired Ambient Time Time Time Furnace Alloy to Soak to Soak Savings Temper- Tem Temper- (hrs.) ature (hrs. ature (deg) (hrs.)

5086 24 2 22 1, 675 2024 19 2 17 1,425 3033 l9 2 17 1,465 5052 24 2 22 1, 520 2024 Clad wll230 Linen..- 25 5 20 1, 150

HEAT TO HOMOGENIZE Homoge- Average Direct Average nization Time to Fired Time Ambient Alloy Temper- Temper- Time to Savings Furnace ature ature Temper- (hrs.) Temper- (deg.) (hrs.) ature ature (hrs.) ((16%) A furnace similar to that shown diagrammatically in FIGS. 3 and 4 was used in the heat treatment of ingots as shown in Table I. With reference to FIGS. 3 and 4, the furnace was constructed with a brick linin 25 and had the following inside dimensions: 45" wide x 6'9 long x 10'9" high. The furnace contained four burners 26, which were located approximately 12 inches from the ingot 11 and 3 feet and 6 feet respectively from the bottom of ingot 11 on each side. In the examples shown in Table I, ingot 11 had the following approximate dimensions: 18" thick x 40" wide x 112" high. The burners 26 were the type known in the trade as North American No. 4832-6, which give a flat flame. With this type of burner, the flame fans out substantially parallel to the inside walls 28 of the furnace so as not to impinge directly on the ingot surfaces 27. Ingot .11 was placed on dolly 29, mounted for movement as on rollers 30, and moved into and out of the furnace by conventional means not shown. In each heat treatment shown in Table I, the furnace was pre-heated for approximately one hour to an ambient temperature of 950 F. before loading the ingot 11. After loading, the gas was not turned on until the surface temperature of the ingot was approximately F. At this point, the ambient temperature is the furnace was approximately 750 F. The flame from the burners 17 was controlled so it did not hit the ingot surface 18 directly.

In the first example, an aluminum alloy 5086 scalped ingot was heat treated in accordance with heat to roll practice wherein the desired rolling temperature is 850 F. The alloy known as 5086 comprises 4.0% magnesium, 0.5% manganese, 0.40% maximum silicon, 0.50% maximum iron, 0.25% maximum zinc, 0.10% maximum copper, 0.05-0.25% chromium, 0.15% maximum titanium with to balance aluminum. In accordance with the practice of this invention, two hours were required to raise the ingot to soak temperature in the direct fired furnace. The average cycle time for a 5086 alloy ingot is approximately 30 hours of which 6 hours are fixed in soak time; therefore, 24 hours are normally required to heat the ingot to soak temperature. Here the time to soak temperature was 2 hours with a time saving of 22 hours resulting therefrom. When inspected before rolling, the ingot was nearly as bright as before it went into the furnace and was free of water stain, heavy surface oxidation and high temperature oxidation.

The next example involved heat treating aluminum alloy 2024 scalped ingot in accordance with heat to roll practice wherein the desired rolling temperature was 850 F. The alloy known as 2024 contains 45% copper, 1.5% magnesium, 0.6% manganese, 0.50% maximum silicon, 0.50% maximum iron, 0.10% maximum chromium, 0.25 maximum zinc, the balance aluminum. The average cycle time in heating 2024 alloy ingot in accordance with heat to roll practice is 20 hours, one hour of which is soak time; therefore, 19 hours are normally required to heat the ingot to soak temperature. When heated to soak temperature in accordance with this invention, 2 hours were required, thus 17 hours of time were saved. When the ingot surface was examined before rolling, it was shiny, free from water stains, heavy surface oxidation and high temperature oxidation.

In the next example shown in Table I, aluminum 3003 alloy scalped ingot was heat treated in a direct fired furnace in accordance with heat to roll practice wherein the desired rolling temperature was 850 F. The aluminum alloy known as 3003 contains 1.2% manganese, 0.6% maximum silicon, 0.7% maximum iron, 0.20% maximum copper, 0.10% maximum zinc, the balance aluminum. The normal cycle time for heating to roll 3003 alloy in soaking pits is 20 hours, one hour of which is soak time. Therefore, 19 hours are normally required to heat the ingot to the soak temperature. When the ingot was heated in accordance with this invention, the time required to heat the ingot to soak temperature was 2 hours. Thus, 17 hours time saving resulted. The ingot, after being heat treated in accordance with this invention, was shiny, free of excessive surface oxide, Water stains and high temperature oxidation. A sample was taken from the surface of the ingot after it had been rolled. The sample was anodized to make any rolled in oxide more apparent. The sample showed good anodizing quality and was free of excessive rolled in oxide.

In the next example shown in Table I, a scalped ingot of aluminum 5052 alloy was heat treated in accordance with heat to roll practice. The aluminum alloy known as 5052 contains 2.5% magnesium, 0.25% chromium, 0.45% maximum silicon and iron, 0.10% maximum copper, 0.10% maximum manganese, 0.10% maximum zinc,

the balance aluminum. The average cycle time for heating 5052 ingot in accordance with heat to roll practice in the soaking pits is 25 hours, one hour of which is fixed for a soak. Therefore, 24 hours are normally required to heat the metal to the soak temperature. When the ingot was heat treated in accordance with this invention in a direct fired furnace, only 2 hours were required to heat the ingot up to soak temperature. Thus, a saving of 22 hours was realized from practicing this invention. The ingot showed a bright surface, free of water staining, heavy surface oxidation, and high temperature oxidation.

The next example shown in Table 1 involved the heat treatment of a scalped 2024 ingot clad with 0.400 of aluminum alloy 1230 liner. The aluminum alloy known as 1230 comprises not more than 0.7% silicon plus iron, 0.10% maximum copper, 0.05% maximum manganese, 0.10% maximum zinc with the balance aluminum. This ingot was heat treated in accordance with heat to roll practice wherein the desired rolling temperature was 850 F. When heat treating clad ingot, the ambient furnace temperature must be kept below the melting point of the cladding material or the cladding will melt as previously discussed. Thus, here the ambient furnace temperature was carried between 1100 and 1200 F. for an average ambient temperature of 1150 F. The normal time required for bringing 2024 clad alloy up to soak temperature in the soaking pit is 25 hours. When such alloy was heat treated in accordance with this invention, only 5 hours were required to bring the ingot up to temperature resulting in a time saving of hours. The composite ingot sealed satisfactorily, was free of water staining, excessive surface oxide, and high temperature oxidation.

In the next example shown in Table I, a scalped ingot of aluminum 5086 alloy was heat treated in accordance with homogenization practice wherein a soak of 6 hours at an average temperature of 950 F. is given the ingot. The composition of aluminum 5086 alloy has previously been given. The average cycle time for heat treating aluminum 5086 alloy ingot in accordance with homogenization practice is approximately hours. When the ingot is heat treated in accordance with this invention, the total cycle time is approximately 8 hours, resulting in a time saving of 22 hours. The saving resulted from the fact that ordinarily it takes 24 hours to bring the ingot up to temperature whereas when heat treated in accordance with this invention, only 2 hours were required to bring the ingot up to temperature. The in Ot was shiny, free of water stain, excessive surface oxidation, and high temperature oxidation.

The final example shown in Table I involved heat treating aluminum 3003 alloy scalped ingot in accordance with homogenization practice wherein it is desired to give the ingot a 9 hour soak at an average temperature of 1125 F. The average cycle time for heat treating aluminum 3003 alloy ingots in accordance with homogenization practice is hours. Nineteen of these hours are consumed by the soak and slow cool time. Thirty-one hours are normally required to get the ingot to soak temperature. When heat treated in accordance with this invention, only 3 hours were required to reach the soak temperature, thus, a time saving of 28 hours resulted. The ingot, when examined, was shiny, free of water stain, heavy surface oxidation, and high temperature oxidation.

It should be noted that in both homogenization exan ples given, the degree of surface oxidation was greater than that encountered on the heat to roll practice examples. This was expected and is due to the longer period of time at which the ingot is held at an elevated temperature. However, the degree of surface oxidation was not excessive and the surface of the ingot in all cases looked as well as, if not better than, ingots that were heat treated in accordance with normal homogenization practice.

It will be understood that various changes, modifications, and alterations may be made in the instant invention without departing from the spirit and scope thereof and, as such, the invention is not to be limited except as by the appended claims wherein:

What is claimed is: 1. The method of heat treating ingots of aluminum and 5 its alloys comprising:

(a) heating the ingot out of contact with combustion product gases until at least the surface temperature of the ingot is above the dew point of the water vapor in the combustion product gases,

(b) thereafter heating the ingot in a direct fired furnace in contact with combustion product gases to the heat treat temperature,

(c) holding the ingot at said temperature in contact with combustion product gases for the desired period of time.

2. The method of heat treating ingots of aluminum and its alloys comprising:

(a) heating the ingot out of contact with combustion produce gases until at least the surface temperature of the ingot is above the dew point of the water vapor in the combustion product gases,

(b) thereafter heating the ingot in a direct fired furnace in contact with combustion product gases to the heat treat temperature,

(c) holding the ingot at said temperature for the desired period of time.

3. The method of heat treating ingots of aluminum and its alloys comprising:

(a) charging the ingot into a furnace zone free of cornbustion product gases and heating the ingot therein until at least the surface temperature of the ingot is above the dew point of the water vapor in the combustion product gases,

(b) passing the ingot into a direct fired furnace zone, disposing the ingot therein out of direct contact with the flame and in contact with combustion product gases and heating the ingot therein to the heat treat temperature,

(c) then passing the ingot into a soaking furnace zone and holding the ingot therein at said temperature for the desired period of time.

4. The method of heat treating clad ingots of aluminum alloys comprising:

(a) heating the clad ingot out of contact with combustion product gases until at least the surface temperature of the clad ingot is above the dew point of the water vapor in said combustion product gases,

(b) thereafter heating the clad ingot in a direct fired furnace in contact with combustion product gases to the heat treat temperature while maintaining the ambient temperature in said direct fired furnace below the melting point of the cladding material.

(c) holding the clad ingot at said temperature for the desired period of time.

5. The method of heat treating ingots of aluminum and its alloys comprising:

(a) heating the ingot out of contact with combustion product gases until at least the surface temperature of the ingot is above the dew point of the water vapor in the combustion product gases,

(b) thereafter heating the ingot to the heat treat temperature in contact with combustion product gases and out of direct contact with flame in a direct fired furnace,

(c) holding the ingot at said temperature for the desired period of time.

6. The method of heat treating ingots of aluminum and its alloys comprising:

(a) heating the ingot out of contact with combustion product gases until at least the surface temperature of the ingot is above the dew point of the Water vapor in the combustion product gases,

(b) thereafter heating the ingot to the heat treat temperature in contact with combustion product gases in a direct fired furnace, the flame from the burner in said furnace being controlled so as to fan out substantially parallel to the furnace walls, and not to impinge directly on the ingot surface,

() holding the ingot at said temperature for the desired period of time.

7. The method of heat treating ingots of aluminum and its alloys comprising:

(a) heating the ingot out of contact with combustion product gases until at least the surface temperature of the ingot is above the dew point of the water vapor in the combustion product gases,

(b) thereafter heating the ingot in a direct gas fired furnace in contact with combustion product gases to the heat treat temperature,

(c) holding the ingot at said temperature for the desired period of time.

8. The method of claim in which said direct fired furnace is gas fired.

9. The method of claim 6 in which Said direct fired furnace is gas fired.

10. The method of heat treating ingots of aluminum and its alloys comprising:

(a) preheating a furnace zone to an ambient temperature above the dew point of water vapor in combustion product gases,

(b) charging the ingot therein and heating the ingot out of contact with combustion product gases until at least the surface temperature of the ingot is above the dew point of water vapor in the combustion product gases,

(0) thereafter heating the ingot in a direct fired furnace zone in contact with combustion product gases to the heat treat temperature,

(d) holding the ingot at said temperature for the desired period of time.

11. Apparatus for heat treating ingots of aluminum and its alloys comprising:

(a) means for heating the ingot out of contact with combustion product gases until at least the surface temperature of the ingot is above the dew point of the water vapor in the combustion product gases,

(b) a direct fired furnace for heating the ingot to the heat treat temperature in contact with combustion product gases,

(0) means for transferring the ingot to the direct fired furnace after the surface temperature of the ingot is above the dew point of the water vapor in the combustion product gases,

(d) and means for holding the ingot at said heat treat temperature for the desired period of time.

12. Apparatus for heat treating ingots of aluminum and its alloys comprising:

(a) means for heating the ingot out of contact with combustion product gases until at least the surface temperature of the ingot is above the dew point of water vapor in said combustion product gases,

(b) a direct fired furnace for heating the ingot to heat treat temperature in contact with combustion product gases, said furnace having means for controlling the flame from its burners adapted to prevent the flame from the burners hitting the ingot surface directly,

(c) means for transferring the ingot to the direct fired furnace after the surface temperature of the ingot is above the dew point of the water vapor in the combustion product gases,

(d) and means for holding the ingot at said heat treat temperature for the desired period. of time.

13. Apparatus for heat treating ingots of aluminum and its alloys comprising:

(a) means for heating the ingot out of contact with combustion product gases until at least the surface temperature of the ingot is above the dew point of water vapor in said combustion product gases,

(b) a direct fired furnace for heating the ingot to heat treat temperature in contact with combustion product gases having burners therein, said burners being of a type giving a flat flame generally parallel to the walls of said furnace,

(c) means for transferring the ingot: to the direct fired furnace after the surface temperature of the ingot is above the dew point of the water vapor in the combustion product gases,

(d) and means for holding the ingot at said heat treat temperature for the desired period of time.

14. Apparatus for heat treating ingots of aluminum and its alloys comprising a circular heating chamber having an entrance and exit, a conveyor mounted for traveling movement through said chamber in a circular path, said chamber being divided into a plurality of zones, gas sealing means between the first and second zones, the first zones having means for heating the ingot out of contact with combustion product gases until at least the surface temperature of the ingot is above the dew point of water vapor in said combustion product gases, the next zones comprising a direct fired furnace for heating the ingot to the heat treat temperature in contact: with combustion product gases, the final zones of said chamber having means for holding the ingot at said temperature for the desired period of time.

15. Apparatus for heat treating ingots of aluminum and its alloys comprising a heating chamber having an entrance and exit, a conveyor mounted for traveling movement through said chamber from said entrance to said exit, said chamber being divided into a plurality of zones, gas sealing means between the first and second zones, the first zones having means for heating the ingot out of con tact with combustion product gases until at least the surface temperature of the ingot is above the dew point of water vapor in said combustion product gases, the next zones comprising a direct fired furnace for heating the ingot to the heat treat temperature in contact with combustion product gases, the final zones of said chamber having means for holding the ingot at said temperature for the desired period of time.

References Cited UNITED STATES PATENTS 991,929 5/1911 Baily 13-2 1,507,665 9/1924 Clark 263-52 1,996,379 5/1935 Keller 266-5X 2,233,474 3/1941 Dretfein 266-536 2,406,554 8/1946 Marsh 263-52 2,595,991 5/1952 Sweeney 148--167X 3,159,387 12/1964 Campbell et a1 263-40 3,197,184 7/1965 Navez et a1 263-28 X FREDERICK L. MATTESON, JR., Primary Examiner.

JOHN J. CAMBY, Examiner. 

1. THE METHOD OF HEAT TREATING INGOTS OF ALUMINUM AND ITS ALLOYS COMPRISING: (A) HEATING THE INGOT OUT OF CONTACT WITH COMBUSTION PRODUCT GASES UNTIL AT LEAST THE SURFACE TEMPERATURE OF THE INGOT IS ABOVE THE DEW POINT OF THE WATER VAPOR IN THE COMBUSTION PRODUCT GASES. (B) THEREAFTER HEATING THE INGOT IN A DIRECT FIRED FURNACE IN CONTACT WITH COMBUSTION PRODUCT GASES TO THE HEAT TREAT TEMPERATURE, (C) HOLDING THE INGOT AT SAID TEMPERATURE IN CONTACT WITH COMBUSTION PRODUCT GASES FOR THE DESIRED PERIOD OF TIME. 